1. Field of the Invention
The embodiment discussed herein is related to a mask inspection apparatus having a function to generate a wide field of view and high definition image and also to an image generation method.
2. Description of the Related Art
In a lithography process of semiconductor manufacturing processes, a pattern formed on a photomask is exposed to light to be transferred onto a wafer by use of an exposure device. If the pattern formed on the photomask has a defect or distortion, such a defect or distortion causes a failure to transfer the pattern at a desired position, a failure to form the pattern into an accurate shape or the like, i.e., causes a reduction in accuracy of the exposure. In order to prevent such a reduction in accuracy of the exposure, an inspection is conducted to find a positional error and a defect in a photomask.
As a method of inspecting photomasks, there is an inspection method using an SEM image of a mask captured with a scanning electron microscope. In the scanning electron microscope, a sample is irradiated with incident electrons while the surface of the sample in an electron beam scanning region is scanned by the incident electrons, and secondary electrons emitted from the sample are acquired through a scintillator. Thereafter, the quantity of the acquired electrons is converted into luminance to acquire SEM image data. Then, the SEM image data is displayed on a display.
For example, an inspection using a line width of a pattern formed on a mask is conducted by the following procedures. A predetermined region of a pattern formed on a photomask is displayed on a display. Then, an electron beam is aimed at and applied onto a measurement point set within the display region. Thereafter, a luminance distribution waveform is acquired on the basis of secondary electrons reflected from the measurement point. Thereafter, pattern edge positions are found by conducting analysis on the luminance distribution waveform to thereby define them as the line width. Whether or not the line width thus found falls within a tolerance range is determined to judge whether the quality of the photomask is good or not.
In addition, there is a mask inspection method in which a mask and a mask model are compared with a result of a transfer simulation onto a wafer. In this mask inspection method, a simulation to find how a pattern is transferred onto a wafer is performed on the basis of an inspection image obtainable from transmitted light and reflected light using a mask. Then, the result of the simulation is compared with a result of a simulation performed to find how the pattern is transferred onto the wafer with a correct mask. It results in inspecting whether or not a defect exists in the pattern on the mask, and so on. This transfer simulation requires a field of view of approximately 10 micron, and the mask model and an SEM image are compared to inspect whether or not a defect exists in the pattern formed on the mask, and so on. The pattern of the entire photomask is reflected to this mask model. Thus, the SEM image for comparison with the mask model is required to be a wide field of view, as well.
In the mask inspection apparatus using the aforementioned scanning electron microscope or the like, however, highly accurate measurement is required. For this reason, in general, an SEM image is acquired by using a limited, narrow field of view with a high magnification. Moreover, in a case of a normal length measurement SEM, scanning with a wide field of view causes aberrations such as astigmatism, field curvature and distortion, and therefore requires dynamic adjustment of such aberrations during the scanning. Therefore, this inspection apparatus not only causes a significant load for correction, but also results in a situation where the aberrations are not sufficiently corrected.
In this respect, Japanese Laid-open Patent Publication No. 2000-294183 describes a technique to acquire a wide field patched image of a sample with an SEM while automatically driving a sample stage at the time of capturing divided SEM images of the sample.
As described above, in order to acquire a wide field SEM image, SEM images are captured in a divided manner and the captured SEM images are patched together.
However, with the technique described in Japanese Publication No. 2000-294183, when moving the sample stage to a divided area, there is no guarantee to move it to the correct position. Thus, even when patched images are captured, there is no guarantee that the images are combined into a single wide field image.
In addition, when the SEM images acquired in a divided manner are patched together, the operator detects target images for joining two areas together, and then combines the two areas in such a way that the two target images are connected to each other. Accordingly, generation of a high definition SEM image requires enormous time and efforts.